1. Field of the Invention
The present invention relates to a thin film transistor (TFT) substrate of a transflective liquid crystal display device and more particularly to simplified process for fabricating a transflective TFT transistor substrate.
2. Description of the Related Art
Liquid crystal display devices are generally classified into two types: the transmissive type in which pictures are displayed using light supplied from a backlight unit, and the reflective type in which pictures are displayed using light reflected from an external source, such as natural light. There is a problem that the power consumption of a backlight unit is high in the transmissive type and the reflective type depends on the external light so as not to be able to display the picture in a dark environment
In order to resolve such a problem, a transflective liquid crystal display device is on the rise, wherein the transflective liquid crystal can be selected to be in a transmissive mode where the backlight unit is used or in a reflective mode where the external light is used. The transflective liquid crystal display device operates in the reflective mode if the external light is sufficient and in the transmissive mode if the external light is not sufficient, thus it might be able to reduce the power consumption more than the transmissive liquid crystal display device and it is not restricted by the external light, which is different from the reflective liquid crystal display device.
Generally, a transflective liquid crystal display panel, as shown in FIG. 1, includes a color filter substrate and a thin film transistor substrate which are bonded together with a liquid crystal layer (not shown) between them, and a backlight unit arranged behind the thin film transistor substrate. Each pixel of the transflective liquid crystal display panel is divided into a reflective area where a reflective electrode 28 is formed, and a transmissive area where the reflective electrode 28 is not formed.
The color filter substrate includes a black matrix (not shown) and a color filter 54 formed on an upper substrate 52, and a common electrode 56 and an alignment film (not shown) formed there over.
The thin film transistor substrate includes a gate line 4 and a data line (not shown) formed on a lower substrate 2 that define each pixel area; a thin film transistor connected to the gate line 4 and the data line; a pixel electrode 32 formed at the pixel area and connected to the thin film transistor; and a reflective electrode 28 formed at a reflective area of each pixel to overlap the pixel electrode.
The thin film transistor includes a gate electrode 6 connected to the gate line 4; a source electrode 16 connected to the data line; a drain electrode 18 facing the source electrode 16; an active layer that overlaps the gate electrode 6 with a gate insulating film 8 there between to form a channel between the source and drain electrodes 16, 18; and an ohmic contact layer 12 to cause the active layer 10 to be in ohmic-contact with the source and drain electrodes 16 and 18. The thin film transistor responds to a scan signal of the gate line 4 to cause a video signal on the data line to be charged and maintained in the pixel electrode 32.
The reflective electrode 28 reflects an external light that is incident through a color filter substrate, toward the color filter substrate. The surface of the organic film 24 formed under the reflective electrode 28 has an embossed or raised shape, therefore the reflective electrode 28, which is formed on top of the organic film, also has an embossed shape. As a result, the reflective efficiency of the reflective electrode 28 increases due to the dispersion effect of the embossed surface.
When a pixel signal is applied to the pixel electrode 32 through the thin film transistor, a potential difference between a common electrode 56 and the pixel electrode 28 is generated. The potential difference causes a liquid crystal having dielectric anisotropy to rotate, thereby controlling the transmissivity of the light that runs through the liquid crystal layer in both the reflective and transmissive areas, thus its brightness is changed in accordance with the video signal.
In this case, a transmission hole 36 is formed in a relatively thick organic film 24 at a transmissive area so that the length of the light path going through the liquid crystal layer is the same in the reflective area as in the transmissive area. As a result, the length of the path RL that ambient light incident to the reflective area travels is the same as the length of the path TL that transmitted light from the backlight unit 60, thus the transmissive efficiency is the same in both the reflective and transmissive modes.
The thin film transistor substrate further includes a storage capacitor connected to the pixel electrode 32 in order to stably maintain the video signal supplied to the pixel electrode 32. The storage capacitor is formed by having a storage upper electrode 20 overlap the gate line 4 with a gate insulating film there between, wherein the storage upper electrode 20 is connected to the pixel electrode 32. The ohmic contact layer 12 and the active layer 10 further overlap under the storage upper electrode 20 in the process.
The thin film transistor substrate further includes a first passivation film 22 between the thin film transistor and the organic film 24; a second passivation film between the organic film 24 and the reflective electrode 28; and a third passivation film 30 between the reflective electrode 28 and the pixel electrode 32. Accordingly, the pixel electrode 32 is connected to the drain electrode 18 and the storage upper electrode 20 through each of the first and second contact holes 34, 38 that penetrate the first to third passivation films 22, 26, 30, an organic film 24 and the reflective electrode 28.
In such a transflective liquid crystal display panel, the thin film transistor substrate includes the semiconductor process and requires a plurality of mask processes, thus its manufacturing process is complicated so that it becomes a material cause for the increase of the liquid crystal display panel manufacturing cost. Hereinafter, a fabricating method of the transflective thin film transistor substrate will be described in reference with FIGS. 2A to 2F.
Referring to FIG. 2A, a gate metal layer is formed on the lower substrate 2 using a deposition method such as sputtering. Subsequently, the gate metal layer is patterned with a first mask using a photolithography process and an etching process, thereby forming the gate pattern including the gate line 4 and the gate electrode 8. The gate metal layer is a single layered or double layered metal such as Al, Mo, Cr.
Next, the gate insulating film 8 is formed on the substrate 2 where the gate pattern is formed, and a source/drain pattern is formed on top thereof using a second mask process as illustrated in FIG. 2B The source/drain pattern includes the data line, the source electrode 16, the drain electrode 18 and the storage upper electrode 20.
The gate insulating film 8, an amorphous silicon layer 10, an amorphous silicon layer with impurities doped thereto 12, and the source/drain metal layer are sequentially formed on the lower substrate 2 where the gate pattern is formed. The gate insulating film 8 is an inorganic insulating material such as silicon oxide SiOx or silicon nitride SiNx, and the source/drain metal layer is a single or double layered metal structure such as Al, Mo and the like.
A photo resist pattern is formed on top of the source/drain metal layer using a second mask and a photolithography process. During this process, a diffractive exposure mask with a diffractive exposure part at a channel part of the thin film transistor is used as the second mask, thus the photo resist pattern of the channel part is made to have a lower height than the source/drain pattern part.
There after, the source/drain metal layer is patterned by wet etching using the photo resist pattern to form the source/drain pattern that includes the data line, the source electrode 16, the drain electrode 18 integrated with the source electrode 16, and the storage upper electrode 20.
Next, an amorphous silicon layer doped with the impurities and an amorphous silicon layer are simultaneously patterned by a dry etching using the same photo resist pattern, thereby forming the ohmic contact layer 12 and the active layer 10.
After removing the photo resist pattern having relatively low height at the channel part by ashing, the source/drain pattern and the ohmic contact layer 12 of the channel part are dry etched. Accordingly, the active part 10 of the channel part is exposed to separate the source electrode 16 from the drain electrode 18. There after, the photo resist pattern remaining on the source/drain pattern is removed using a strip process.
Referring to FIG. 2C, a first passivation film 22 is formed on the gate insulating film 8 where the source/drain pattern is formed, and an organic film 24 is formed on top thereof using a third mask process, such that the organic film 24 has first and second initial contact holes 34, 38 and a transmission hole 36 with an embossed shaped surface.
The first passivation film 22 and the organic film 24 are sequentially formed on the gate insulating film 8 where the source/drain pattern is formed. The first passivation film 22 is of the same inorganic insulating material as the gate insulating film 8, and the organic film 24 is of a photosensitive organic material such as acrylic resin.
Then, the organic film 24 is patterned using the third mask, thereby forming first and second open holes 35, 37 and the transmission hole 36 which penetrate the organic film 24 in correspondence to the transmissive part of the third mask. The third mask has a structure where a shielding part and a diffractive exposure part repeat at the rest area except for the transmissive part. The organic film 24 remaining in correspondence thereto is patterned to have a structure that a shielding area (projected part) and a diffractive exposure area (groove part) having a stepped difference are repeated. Subsequently, the organic film 24 where the projected part and the groove part are repeated is cured so that the surface of the organic film 24 has the embossed shape.
Referring to FIG. 2D, a second passivation film 26 is formed on the organic film 24, and the reflective electrode 28 is formed on top thereof using a fourth mask process. The second passivation film 26 and the reflective metal layer are deposited to maintain their embossed shape on top of the organic film 24 that has the same embossed surface. The second passivation film 26 is an inorganic insulating material such as the first passivation film 22, and the reflective metal layer is a metal with high reflectivity such as AlNd.
Subsequently, the reflective metal layer is patterned using a fourth mask and etching process, thus the reflective electrode 28 is formed, wherein the reflective electrode is independent every pixel and is opened at the transmission hole 36 and the first and second open holes 35, 37 of the organic film 24.
Referring to FIG. 2E, a third passivation film 30 covering the reflective electrode 28 is formed using a fifth mask process, and first and second contact holes 34, 38 penetrating the first to third passivation films 22, 26, 30 are formed. The third passivation film 30 the reflective electrode 28 and the first and second contact holes 34, 38 are formed with the fifth mask using photolithography and etching processes, such that the first and second contact holes 34, 38 penetrate the first to third passivation films 22, 26, 30 at the first and second open holes 35, 37 of the organic film 24. The first and second contact holes 34, 38 each expose the drain electrode 18 and the storage upper electrode 20. The third passivation film is of the same inorganic insulating material as the second passivation film.
Referring to FIG. 2F, a pixel electrode 32 is formed on the third passivation film 30 by use of a sixth mask process. More specifically, a transparent conductive layer is formed on the third passivation film 30 using a deposition method such as sputtering, and the transparent conductive layer is patterned by the photolithography process using a sixth mask and the etching process to form the pixel electrode at each pixel area. The pixel electrode 32 is connected to the drain electrode 18 and the storage upper electrode 20 through the first and second contact holes 34 and 38. The transparent conductive layer is of indium tin oxide ITO.
Accordingly, the related art transflective thin film transistor substrate is formed using six 6 different mask processes, thus there is a disadvantage that its manufacturing process is complicated. Further, the margin of the first and second contact holes 34, 38 should be secured sufficiently in order for the pixel electrode 32 to be connected to the drain electrode 18 and the storage upper electrode 20 in the related art transflective thin film transistor substrate. Because of this, there is a disadvantage that the aperture ratio of the transmissive area is reduced.